Traditional methods employing guided waves for inspecting pipes comprise two stages. The first stage is illustrated in FIG. 1 where a pipe section 20 has an array of transducers 22 mounted over the circumference and an axially propagating guided wave 24. In the first stage, an axially propagating, unfocussed, guided wave is generated by exciting the transducers in the array of transducers 22 simultaneously so that the energy of the wave packet is distributed around the circumference of pipe. This method is limited to detecting flaws that can be attributed to material loss or flaws with circumferential extent (see Hardie F., “Evaluation of the effectiveness of non-destructive testing screening methods for in-service inspection,” Report for the Health and Security Officer, UK, 2009, pages 29-30).
For the first stage, the disadvantage of traditional methods is that the flaws must necessarily be either material loss type or circumferentially oriented because of the direction of wave propagation. Because the energy of the wave is distributed throughout the circumference, the intensity of the wave is low leading to its premature dissipation when pipe is carrying fluid or is submerged or buried (see Hardie F, “Evaluation of the effectiveness of non-destructive testing screening methods for in-service inspection,” Report for the Health and Security Officer, UK, 2009, pages 29-30).
The second stage can overcome the limitations of the first stage by providing high intensity ultrasound at the region of interest. The second stage is illustrated in FIG. 2 where a pipe section 26 has an array of transducers 28 mounted over the circumference. A focused waveguide 30 that focuses rays 32 emitted from the array of transducers 28 just before the focal point. One of the disadvantages of the focusing method, when compared to the unfocused guided wave method, is that the inspection is point-by-point which, can be time consuming. The time required for capture can be on the order of 1 ms or more per point for the best case scenario (see, e.g., Li, J. et al “Angular-profile tuning of guided waves in hollow cylinders using a circumferential phased array,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on 49.12, 2002, pages 1720-1729; see also, e.g., Sun, Z. et al., “Flexural torsional guided wave mechanics and focusing in pipe,” Journal of pressure vessel technology 127.4, 2005, pages 471-478).
Focusing is typically achieved by actuating the array of sensors with time delayed signals through a system, such as the embodiment of a system depicted in FIG. 3. The system includes an ultrasonic transducer array 34 mounted on a pipe, a multichannel preamplifier 36 that receives rays emitted by the ultrasonic transducer array 34, a multichannel analog to digital converter 38, and a computer/controller 40. The system also includes a pulse generator 42 controlled by the computer/controller 40 with variable time delays, amplitudes, frequencies and cycles across the channels. Control signals are sent from the pulse generator 42 to the ultrasonic transducer array 34 to control the emission of rays from the ultrasonic transducer array 34. The system also optionally includes an ultrasonic receiver array 44 mounted on the pipe.
Other disadvantages of the focusing method relate to factors involving the hardware depicted in FIG. 3, such as latency of the hardware when settings are changed to shift the point of focus. The circumferential location of the focal point is changed by switching the order in which the transducer array elements are excited (see Sun, Z. et al., “Flexural torsional guided wave mechanics and focusing in pipe,” Journal of pressure vessel technology 127.4, 2005, pages 1724 and 1727). Another disadvantage to the guided wave focusing method is the limited a circumferential resolution based on the number of elements in the array. Another disadvantage of the traditional focusing method that can be deduced from literature is that it is most sensitive to flaws that have a circumferential extent because the focused beam is formed by symmetric contribution from all the transducers, as illustrated by the rays 32 depicted in FIG. 2.
The second stage is typically used to size and find the circumferential location of the flaw. Focused guided waves can also be optionally used to generate a C-scan or a detailed map of a pipe by inspecting it point-by-point as the focal point of the wave axially and circumferentially shifted by manipulating the transducer elements' excitation.
All of the above methods are ineffective when there are two flaws with one flaw hidden behind the shadow of another, as illustrated in FIG. 4. In FIG. 4, a pipe 46 includes a transducer array 48 that produce incident rays 50 corresponding to the focused guided wave generated by the transducer array 48 in the pipe 46. The pipe includes a first flaw 54 and a second flaw 56. The first flaw 54 is bigger than the second flaw 56 and the second flaw is located in a shadow of the first flaw 54. The incident rays 50 are reflected by the first flaw 54 and reflection rays 52 return away from the first flaw 54. The first flaw 54 is larger than the second flaw 56 and the second flaw 56 is located in a shadow of the first flaw 54. As a result, as shown in FIG. 4, the incident rays 50 and the reflection rays 52 not hit the second flaw 56 and the second flaw 56 cannot be detected.
Mixing of the ultrasound array parameters, namely, time delays and amplitude variation is known as apodization. Apodization has been suggested in literature as a method for improving spot size of the focused waves. The primary aim of apodization thus far has been to reduce the so called Fourier noise caused due to the finite geometrical extent of an array of transducers. Further, apodization is performed without taking into account the fact that the minimum time delay offered by hardware limits the frequency at which good quality beam forming is achieved. Recently, it was shown that time delays can be completely replaced by amplitude variation across the transducer elements (see Kannajosyula, H., et al., “Amplitude controlled array transducers for mode selection and beam steering of guided waves in plates,” Review of Progress in Quantitative Nondestructive Evaluation: Volume 32, American Institute of Physics, 2013).
By virtue of the principle of reciprocity, theories developed for beam steering have enabled the development of post-processing algorithms in literature for tools that employ an array of ultrasonic sensors each of which discretely transmit and/or receive ultrasonic guided wave signals in the structure. Such post-processing algorithms are able to filter flaw signatures from the received data and thereby image the structure. Such algorithms are commonly referred to as synthetic phased array method and tools employing such algorithms have been referred to as ultrasonic radar or ultrasonic guided wave radar.
Unfocused beam forming in plates has been shown to be possible (see, e.g., Kannajosyula, H., et al., “Amplitude controlled array transducers for mode selection and beam steering of guided waves in plates,” Review of Progress in Quantitative Nondestructive Evaluation: Volume 32, American Institute of Physics. 2013). In principle, wave propagation in a pipe of very large diameter and small wall thickness will be similar to that in a plate. However, this may not necessarily be true for pipes of smaller diameters. Hence extension of beam forming technique used in plates to beam forming in pipes is not straightforward. Conversely, a method for focused beam forming in plates has not yet been developed in literature. Theory used for pipes can be extended to plates by modeling plates as very large diameter pipes. However; current theory appears to need further development for this to be possible.